Although thermoplastic resins and their applications are well known, reinforced resins are relatively new and have significant advantages over pure, resinous composites. Fiber reinforcement toughens and stiffens the resin to produce high performance products. At the same time, processing is not seriously hindered because the reinforced resin maintains its thermoplastic character. For example, a sheet of fiber reinforced resin can be heated, stamped into a desired shape by appropriate dies, reheated and restamped to alter the shape. In contrast, a thermosetting resin cannot be reshaped, once it is fully cured by heating. Thermoplastic resins, however, generally exhibit poor solvent resistance, and this deficiency has severely limited their use. For example, reinforced thermoplastic resin circuit boards of conventional design cannot be cleaned by solvents commonly used in the manufacture of circuit boards. Hydraulic fluids and cleaning fluids in aircraft limit adoption of conventional thermoplastic resins unless their solvent resistance can be improved.
Recently, chemists have sought to synthesize oligomers for high performance advanced composites suitable for aerospace applications. These composites should exhibit solvent resistance, be tough, impact resistant, and strong, be easy to process, and be thermoplastic. Oligomers and composites that have thermo-oxidative stability, and, accordingly can be used at elevated temperatures, are particularly desirable.
While epoxy-based composites are suitable for many applications, their brittle nature and susceptibility to degradation make them inadequate for many aerospace applications, especially those applications which require thermally stable, tough composites. Accordingly, research has recently focused upon polyimide composites to achieve an acceptable balance between thermal stability, solvent resistance, and toughness. The maximum use temperatures of conventional polyimide composites, such as PMR-15, are still only about 600.degree.-625.degree. F., since they have glass transition temperatures of about 690.degree. F.
Linear polysulfone, polyether sulfone, polyester, and polyamide systems are also known, but each of these systems fails to provide as high thermal stability as is required in some aerospace applications.
There has been a progression of polyimide sulfone compounds synthesized to provide unique properties or combinations of properties. For example, Kwiatkowski and Brode synthesized maleic-capped, linear polyarylimides as disclosed in U.S. Pat. No. 3,839,287. Holub and Evans synthesized maleic- or nadic-capped, imido-substituted polyester compositions as disclosed in U.S. Pat. No. 3,729,446. We synthesized thermally stable polysulfone oligomers as disclosed in U.S. Pat. No. 4,476,184 or 4,536,559, and have continued to make advances with polyetherimidesulfones, polybenzoxazolesulfones (i.e., heterocycles), polybutadienesulfones, and "star" or "star-burst" multidimensional oligomers. We have shown surprisingly high glass transition temperatures and desirable physical properties in many of these oligomers and their composites, without losing ease of processing.
Multidimensional oligomers, such as disclosed in our copending application U.S. application Ser. Nos. 726,258; 810,817; and 000,605, are easier to process than many other advanced composite oligomers since they can be handled at lower temperatures. Upon curing, however, the unsaturated phenylimide end caps crosslink so that the thermal resistance of the resulting composite is markedly increased with only a minor loss of stiffness, matrix stress transfer (impact resistance), toughness, elasticity, and other mechanical properties. Glass transition temperatures above 950.degree. F. are achievable.
Commercial polyesters, when combined with well-known diluents, such as styrene, do not exhibit satisfactory thermal and oxidative resistance to be useful for aircraft or aerospace applications. Polyarylesters are unsatisfactory, also, since the resins often are semicrystalline which makes them insoluble in laminating solvents, intractable in fusion, and subject to shrinking or warping during composite fabrication. Those polyarylesters that are soluble in conventional laminating solvents remain so in composite form, thereby limiting their usefulness in structural composites. The high concentration of ester groups contributes to resin strength and tenacity, but also makes the resin susceptible to the damaging effects of water absorption. High moisture absorption by commercial polyesters can lead to distortion of the composite when it is loaded at elevated temperature.
High performance, aerospace, polyester advanced composites, however, can be prepared using crosslinkable, endcapped polyester imide ether sulfone oligomers that have an acceptable combination of solvent resistance, toughness, impact resistance, strength, ease of processing, formability, and thermal resistance. By including Schiff base (--CH.dbd.N--), imidazole, thiazole, or oxazole linkages in the oligomer chain, the linear, advanced composites formed with polyester oligomers of our copending application U.S. application Ser. No. 726,259 can have semiconductive or conductive properties when appropriately doped.
Conductive and semiconductive plastics have been extensively studied (see, e.g., U.S. Pat. Nos. 4,375,427; 4,338,222; 3,966,987; 4,344,869; and 4,344,870), but these polymers do not possess the blend of properties which are essential for aerospace applications. That is, the conductive polymers do not possess the blend of (1) toughness, (2) stiffness, (3) elasticity, (4) ease of processing, (5) impact resistance (and other matrix stress transfer capabilities), (6) retention of properties (over a broad range of temperatures), and (7) high temperature resistance that is desirable on aerospace advanced composites. These prior art composites are often too brittle.
Thermally stable multidimensional oligomers having semiconductive or conductive properties when doped with suitable dopants are also known and are described in our copending applications (including U.S. application Ser. No. 773,381 to Lubowitz, Sheppard, and Torre). The linear arms of the oligomers contain conductive linkages, such as Schiff base (--N.dbd.CH--) linkages, between aromatic groups. Sulfone and ether linkages are interspersed in the arms. Each arm is terminated with a mono- or difunctional end cap (i.e., a radical having one or two crosslinking sites) to allow controlled crosslinking upon heat-induced or chemically-induced curing.
Polyamides of this same general type are described in our copending patent application U.S. application Ser. No. 061,938; polyetherimides, in U.S. application Ser. No. 016,703; and polyamideimides, in U.S. application Ser. No. 092,740.